Question

So, this question has been asked before, but I wanted a question with some of those key words in the title.

The issue is simple: How can I have a templated class, such that for each instance of the template – but not each instance of the class – there is a unique, numerical identifier?

That is, a way to differentiate:

foo<int> f1;
foo<char> f2;
classID(f1) != classID(f2);

but,

foo<int> f3;
foo<int> f4;
classID(f3) == classID(f4);

Related to:

in C++, how to use a singleton to ensure that each class has a unique integral ID?

Assigning Unique Numerical Identifiers to Instances of a Templated Class

Answer 1

template<class T>
class Base
{
public:
    static void classID(){}
private:
    T* t;
};

int main()
{
    Base<int> foo;
    Base<int> foo2;
    Base<char> foo3;

    /*
    unsigned int i  = reinterpret_cast<unsigned int>(Base<int>::classID);
    unsigned int ii = reinterpret_cast<unsigned int>(Base<char>::classID);
    unsigned int iii = reinterpret_cast<unsigned int>(Base<int>::classID);
    /*/
    unsigned int i  = reinterpret_cast<unsigned int>(foo.classID);
    unsigned int ii  = reinterpret_cast<unsigned int>(foo2.classID);
    unsigned int iii  = reinterpret_cast<unsigned int>(foo3.classID);
    //*/

    return ((i != ii) + (i <= ii) + (i >= ii)) == 2;
}

That’s how! It’s lightweight, super easy, and doesn’t use RTTI, although it uses the ridiculously unsafe reinterpret_cast.

Although, maybe I’m missing something?